Tuning Stilbene Photochemistry by Fluorination: State Reordering Leads to Sudden Polarization near the Franck–Condon Region

Abstract

Spontaneous polarization of a nonpolar molecule upon photoexcitation (the sudden polarization effect) earlier discussed for 90°-twisted alkenes is observed and calculated for planar ring-fluorinated stilbenes, trans-2,3,5,6,2',3',5',6'-octofluorostilbene (tF2356) and trans-2,3,4,5,6,2',3',4',5',6'-decafluorostilbene (tF23456). Due to the fluorination, Franck-Condon states S and S are dominated by the quasi-degenerate HOMO-1 → LUMO and HOMO-2 → LUMO excitations, while their interaction gives rise to a symmetry-broken zwitterionic S state. After optical excitation of tF2356, one observes an ultrafast (∼0.06 ps) evolution that reflects relaxation from initial nonpolar S to long-lived (1.3 ns in n-hexane and 3.4 ns in acetonitrile) polar S. The polarity of S is evidenced by a solvatochromic shift of its fluorescence band. The experimental results provide a sensitive test for quantum-chemical calculations. In particular, our calculations agree with the experiment, and raise concerns about the applicability of the common TDDFT approach to relatively simple stilbenic systems.

Primed for Efficient Motion: Ultrafast Excited State Dynamics and Optical Manipulation of a Four Stage Rotary Molecular Motor

“…The strong dependence of the lifetime of this state on solvent polarity (Table 1) is consistent with formation of state with significant rotation around the central double bond and/ or pyramidalization of the carbons. 36 Excitation at 269 nm is followed by a similar cascade through excited states with similar rate constants (EADS summarized in Figure S8). However, the net positive signal between 450 and 550 nm for all time delays demonstrates branching between a state with excited state absorption in this region and the bright state responsible for stimulated emission.…”

Type‐I Energy Level Alignment at the PTCDA—Monolayer MoS2 Interface Promotes Resonance Energy Transfer and Luminescence Enhancement

“…TA Spectroscopy: The TA setup with applications has been described elsewhere. [60][61][62] Briefly, TA spectra are recorded with 8-12 forward and backward pump-probe scans, with 0.02, 0.2, and 2 ps steps. The pumpprobe pulse cross-correlation is 100 fs.…”

Ultrafast Dynamics of Encapsulated Molecules Reveals New Insight on the Photoisomerization Mechanism for Azobenzenes

“…The isomerization mechanism for stilbene has been well-studied in solution. ,, After π → π* excitation to S 1 , stilbene molecules cross a small barrier along the torsional rotation coordinate before reaching a perpendicular geometry, where there is a conical intersection (CI) that brings the molecule back to the ground electronic state . The excited-state lifetime for many stilbene derivatives is on the order of tens of picoseconds, depending on the solvent, and has been successfully described either in terms of solvent viscosity inhibiting the relative rotation of the phenyl rings or with a solvent stabilization model where the polarity of the solvent affects the activation energy to reach the CI at perpendicular geometry. , Either way, the longer excited-state lifetime for 4PrSt@OA 2 is most likely due to restricted rotation around the C=C bond inside the capsule.…”

“…state lifetime for many stilbene derivatives is on the order of tens of picoseconds, depending on the solvent, and has been successfully described either in terms of solvent viscosity inhibiting the relative rotation of the phenyl rings or with a solvent stabilization model where the polarity of the solvent affects the activation energy to reach the CI at perpendicular geometry. 29,40 Either way, the longer excited-state lifetime for 4PrSt@OA 2 is most likely due to restricted rotation around the C=C bond inside the capsule. Passage through the CI determines the branching between trans and cis isomers when molecules return to the ground state.…”